Generally, the window glass for a building serves as a large outlet or inlet of heat, and, for example, when the inside of a building is heated in winter, about 48% of the heat of the inside is taken away from the window, and, when the inside of a building is cooled in summer, about 71% of the heat of the inside comes from the outside through the window. A similar phenomenon occurs in vehicles for movement and the like. Particularly, in an automobile, the ratio of the window glass to the space is larger than that in a building structure, and further an automobile hardly avoids exposure to sunlight, and hence the temperature of the interior of the automobile in a certain environment, e.g., under the blazing sun is likely to become extremely high.
An example of measurement made in an environment of summer in Japan shows that the temperature in a parked automobile is close to 70° C. With respect to the temperature of the trim of the interior of the automobile, the temperature of the upper surface of an instrument panel is increased to about 100° C., and the temperature of a ceiling is increased to about 70° C. Further, even when using a ventilating or cooling apparatus, the trim is not easily lowered in temperature and continues radiating heat toward a driver and a passenger(s) for a long time to make them feel uncomfortable.
As a technique for solving the above problems, there has been developed a dimming glass capable of controlling the transmission of light and heat through it. There is a dimming glass using a dimming element, and examples of such dimming elements include: 1) an electrochromic element using a material which is reversibly changed in transmittance by applying a current or a voltage; 2) a thermochromic element using a material which is changed in transmittance due to the temperature; and 3) a gaschromic element using a material which is changed in transmittance by controlling the atmosphere gas.
Among these elements, the electrochromic element can be electrically controlled with respect to the transmission state of light and heat, and can be adjusted to a transmission state of light and heat according to the intention of a user, and therefore the electrochromic element is very suitable for a dimming material applied to glass for buildings and vehicles. Further, the electrochromic element can maintain its optical properties in a certain state even when no current or voltage is applied to the element, making it possible to reduce the energy used.
There is an electrochromic element of a type such that a part of the materials constituting the element is in a liquid state, and the electrochromic element of this type is required to have a structure which prevents the liquid material from leaking from the element. Buildings and vehicles are assumed to be used for a long term, and the leakage of the liquid material can be prevented over a long term, but this causes the cost to be increased, and therefore it is desired that all the materials constituting the electrochromic element are solid materials.
The conventionally known electrochromic elements using a solid material, such as tungsten oxide, have a principle that the dimming material absorbs a light to achieve dimming. Specifically, the penetration of heat in the form of a light to the interior is suppressed by the absorption of the light. For this reason, when the dimming material having such a dimming principle is employed, there is a problem in that the dimming material has heat due to the absorption of the light, so that the heat of the dimming material is re-radiated to the interior as radiation heat.
As a method for solving this problem, a method in which dimming is performed not by absorbing a light but by reflecting a light is considered. In other words, by using a reflective dimming material which reversibly switches from the mirror state to the transparent state, the penetration of heat to the interior due to the absorption of heat by the dimming material can be prevented.
With respect to the reflective dimming type electrochromic element having such properties, there has been disclosed, for example, an electrochromic element having a reflective dimming layer comprising an alloy of a rare earth metal and magnesium and a hydride thereof, a hydrogen ion conducting, transparent oxide protective layer, an anhydrous solid electrolyte layer, and an ion storage layer, which layers are stacked on one another (see, for example, patent document 1).
The reflective dimming layer has a function to control the reflectance of the electrochromic element, and changes the reflectance by giving or receiving hydrogen ions. The oxide protective layer comprises a compound having hydrogen ion conducting properties, e.g., an oxide, such as niobium oxide, vanadium oxide, or tantalum oxide, or a fluoride, such as magnesium fluoride or lead fluoride, and prevents the reflective dimming layer from suffering oxidation.
The ion storage layer stores hydrogen ions used for controlling the reflectance. When a voltage is applied to the dimming glass, hydrogen ions move from the ion storage layer to the reflective dimming layer through the solid electrolyte layer and oxide protective layer, changing the reflectance of the reflective dimming layer. When a voltage is applied in the opposite direction, hydrogen ions are discharged from the reflective dimming layer, bringing the reflectance of the reflective dimming layer back into the original state. This element, however, uses an expensive rare earth metal in the reflective dimming layer, and therefore, from the viewpoint of the cost, the application of this element to an increased area is difficult.
As a reflective dimming element using an inexpensive and more practical material in the reflective dimming layer, for example, there has been proposed an element having Mg2Ni as a reflective dimming layer and palladium or platinum as a catalyst layer which are stacked on one another (see, for example, patent document 2). However, the material of this type in the transparent state has such a low transmittance that the material cannot be practically used.
The dimming mirror glass using a magnesium-nickel alloy thin film, which is developed by a part of the present inventors, is of a gaschromic system using hydrogen gas, and has a visible light transmittance of about 50%, which has remarkably improved, as compared to 20% of the conventionally reported element using Mg2Ni, and can be possibly put into a practical use soon (see, for example, patent document 3).
Further, with respect to the all-solid-state dimming mirror element using a magnesium-nickel alloy thin film, there has been proposed an all-solid-state dimming mirror light switch having an ion storage layer, a solid electrolyte layer, and a magnesium-nickel alloy as a reflective dimming element stacked on a transparent substrate (see, for example, patent document 4).
The all-solid-state dimming mirror light switch using a magnesium-nickel alloy thin film has a problem in that the switch in the transmission state is colored pale yellow and is not in a completely colorless and transparent state. In view of the transparency of this element in the transmission state, the present inventors have proposed an all-solid-state dimming mirror light switch using a magnesium-titanium alloy thin film or a magnesium-niobium alloy thin film, which exhibits an almost colorless and transparent state when it is in the transmission state (see, for example, patent documents 5 and 6).
However, with respect to the repeated use, this element has a disadvantage in that the element can achieve switching 1,000 times or more, but the resultant element suffers deterioration and thus cannot revert to the reflection state. One of the causes of this deterioration suggests that as the switching is repeated, the reflective dimming layer component and catalyst layer component are gradually diffused through the solid electrolyte layer (see, for example, non-patent document 1).
For removing the above disadvantage, there has been proposed an all-solid-state dimming mirror light switch that can achieve repeatedly switching increased times, in which an aluminum thin film is used as a buffer layer for the purpose of preventing the constituents of the layers from diffusing through the all-solid-state reflective dimming electrochromic element using a magnesium alloy thin film (see, for example, patent document 7).
It has been considered that the reflective dimming layer constituting the surface of the element is in contact with air immediately after the preparation of the element, and hence a thin layer of magnesium oxide is formed on the surface of the reflective dimming layer and this oxide layer functions as a passive layer. However, when the element was maintained in air for a long term, which was performed as an example of environmental test, a phenomenon in which the element lost the dimming properties was observed, and therefore thorough studies were made on the mechanism of deterioration of the element.
As a result, the surface magnesium oxide layer had a so poor function as a passive layer that rapid deterioration of the layer was observed in a high humidity atmosphere. One of the causes of this deterioration suggests that the oxygen and humidity in air cause the reflective dimming layer to change to an oxide or hydroxide (see, for example, non-patent document 2). Therefore, in this technical field, the development of an all-solid-state reflective dimming electrochromic element having high durability and being almost free of environmental deterioration has been strongly desired.